The invention relates generally to inorganic membranes and more particularly to inorganic membranes synthesized from silicone material.
The use of membranes to separate mixtures is becoming a favored technique for effecting separations. Membrane separations tend to require less energy than competing techniques, such as distillation processes. The use of membranes can also be less costly and more simple to implement.
Inorganic membranes are presently involved with only a small part of the industrial separations market and are typically more expensive to manufacture than organic membranes. Although organic membranes tend to be limited to operating temperatures from 100.degree. to 200.degree. C., inorganic membranes can be functional at temperatures above 1000.degree. C. Inorganic membranes have found many industrial applications in the food and beverage industry. Other applications include the concentration of biomolecules from fermentation liqueurs, separation of gases and purification of polluted water and air.
Various techniques have been employed to form inorganic membranes. Examples include laser drilling and the use of sol gel technology. Sol gels are formed through the acidic or basic catalysis of the hydrolysis of metal or semi-metal alkoxides. The gel can be dried and fired to yield amorphous and ceramic-type materials. Thin films of inorganic material can also be formed on ceramic membranes that have large pores to yield a composite membrane including an inorganic thin film on a porous ceramic support.
There have been several attempts at forming inorganic membranes including zeolitic material. Zeolites are crystalline material that contain silicon oxides and aluminum oxides. Zeolites are formed with a three dimensional structure in which tetrahedra of primarily SiO.sub.4 and AlO.sub.4 are crosslinked by sharing oxygen atoms whereby the ratio of Si to O atoms is 1:2. Depending on the manner of bonding, the crystal structures can have various ring sizes containing eight, ten or twelve metal/semi-metal atoms. This presence of rings provides a crystal structure with pores and chambers that are of molecular size.
The presence of these chambers promotes the separation of molecules according to their size or their affinity to the zeolitic surfaces. For example, columns of powdered zeolite crystals can be used in the chromatography separation of alkane mixtures by separating branched and straight chain molecules. Gases can be separated with pressure swing absorption techniques. These are currently being used to separate oxygen from air for use by asthma patients. (Ruthyen, D. M., Chemical Engineering Progress, 42, Feb. 1988).
Early attempts at forming zeolitic membranes involved the incorporation of zeolite crystals into organic membranes. This is discussed in U.S. Pat. No. 4,740,219, the contents of which are incorporated herein by reference. Although these membranes have advantages over organic membranes, they lack many of the other advantages of inorganic membranes. For example, these membranes lack suitable temperature stability.
The preparation of inorganic membranes including zeolitic material is also described in Sano, T., et al., Zeolites, Vol. 11, pp. 842-845 (1991). The article describes forming a gel and heating the gel both with and without stirring. However, it has been found that when such gel synthesis processes are conducted, the resulting product can include significant amounts of zeolite in powder form.
The growth of zeolitic material on glass is described in Canadian Patent No. 1,235,684, which describes using the glass as the silica source for the zeolite crystal structure. However, the product formed in accordance with the methodology disclosed in this patent is most likely an agglomeration of thin flat ZSM-5 zeolitic crystals and not a zeolitic thin film or a film of interlocking crystals.
It has also been proposed to treat porous glass with a mixture of sol gel and other reagents that are appropriate for forming zeolite-A. The resulting zeolite product can provide high separation factors for the ethanol/water separation. This has been reported in Ishikawa, A, J. Chem. Soc., Chem. Commun., 764 (1989). Films of an agglomeration of zeolitic crystals have been observed to form on the walls of autoclaves during the growth of zeolites from gels. These agglomeration films are generally formed in combination with discrete crystals (powders) of zeolites. Membranes formed of such agglomeration films are described in U.S. Pat. No. 5,019,263. The contents of U.S. Pat. No. 5,019,263 and the Ishikawa article are incorporated herein by reference.
Accordingly, conventional methods of forming inorganic membranes having a uniform and suitable structure have been less than completely satisfactory. Similarly, prior art methodology has not achieved a continuous thin film zeolite or a film of interlocking zeolite crystals. Rather, known methodology has achieved an agglomeration of zeolitic powder or a coating of zeolitic crystals on the walls of a ceramic or glass substrate. It is therefore desirable to provide an improved method of producing an improved membrane.